1. Field of the Invention
The present invention relates to a straddled electric vehicle and an electric power unit which includes an electric motor.
2. Description of the Related Art
A two-wheeled electric vehicle is a kind of straddled electric vehicle whose driving source is an electric motor. The electric motor rotates with the electric power that is supplied from a battery which is mounted on the two-wheeled electric vehicle, for example, such that the two-wheeled electric vehicle is able to travel.
In two-wheeled electric vehicles, the output power that is required of the electric motor is increasing from year to year. With increasing output power, the weight and volume of an electric motor has also been increasing, thus making the layout difficult within the vehicle which houses the electric motor.
Generally speaking, an electric motor is mounted on a two-wheeled electric vehicle in such a manner that its output shaft is oriented in the vehicle width direction (the right-left direction of the vehicle). This manner of disposition has a problem in that, as the electric motor becomes larger in size along the direction that the output shaft extends, the electric motor may protrude in the vehicle width direction.
One possible way of disposing the electric motor may be so that the output shaft of the electric motor is oriented in the front-rear direction of the vehicle. This will prevent the electric motor from protruding in the vehicle width direction even when it has a large size along the direction that the output shaft of the electric motor extends.
Japanese Laid-Open Patent Publication No. 2004-210072 discloses a two-wheeled electric vehicle in which an electric motor is disposed so that its output shaft is oriented in the front-rear direction of the vehicle. The two-wheeled electric vehicle disclosed in Japanese Laid-Open Patent Publication No. 2004-210072 adopts a secondary speed-reducing mechanism of a shaft-drive type.
The electric motor generates rotation around a rotation axis which is oriented in the front-rear direction of the vehicle. The rotation generated by the electric motor is transmitted to a primary speed-reducing mechanism. In the primary speed-reducing mechanism, rotation is transmitted from a gear having fewer teeth to a gear having more teeth, such that the rotation is decelerated. These gears of the primary speed-reducing mechanism each rotate around a rotation axis which is oriented in the front-rear direction of the vehicle. The rotation, having been decelerated in the primary speed-reducing mechanism, is transmitted to a propeller shaft that extends along the front-rear direction of the vehicle.
The rotation axis of the wheel shaft of the rear wheel defining a drive wheel is oriented in the vehicle width direction. At the rear end of the propeller shaft, a drive bevel gear is provided. A driven bevel gear is provided on the wheel shaft of the rear wheel. The drive bevel gear and the driven bevel gear are meshed with each other on a side of the rear wheel. The drive bevel gear and the driven bevel gear convert the rotation of the propeller shaft whose rotation axis is oriented in the front-rear direction of the vehicle into rotation whose rotation axis is oriented in the vehicle width direction, and transmit it to the wheel shaft of the rear wheel. As a result of this, rotation of the electric motor is transmitted to the rear wheel, such that the rear wheel rotates.
In the primary speed-reducing mechanism, deceleration of rotation is caused as the rotation is transmitted from a gear having fewer teeth to a gear having more teeth. A gear with a larger number of teeth is adopted in order to increase the deceleration ratio; however, as the number of teeth increases, the diameter of the gear also increases.
The two-wheeled electric vehicle disclosed in Japanese Laid-Open Patent Publication No. 2004-210072 is a vehicle which is meant for the young, and thus is not required to provide a large torque. In such a vehicle, the deceleration ratio does not need to be large, and therefore a gear with a large diameter does not need to be provided in the primary speed-reducing mechanism.
However, as the required torque increases, the deceleration ratio needs to become larger. Japanese Laid-Open Patent Publication No. 2013-209079 discloses a two-wheeled electric vehicle in which an electric motor is disposed so that the output shaft of the electric motor is oriented in the front-rear direction of the vehicle. The two-wheeled electric vehicle disclosed in Japanese Laid-Open Patent Publication No. 2013-209079 adopts a secondary speed-reducing mechanism of a shaft-drive type, and via a transmission path of motive power similar to that in Japanese Laid-Open Patent Publication No. 2004-210072, rotation of the electric motor is transmitted to the rear wheel. In the two-wheeled electric vehicle disclosed in Japanese Laid-Open Patent Publication No. 2013-209079, in order to attain a large deceleration ratio, a gear with a large number of teeth and a large diameter is provided in the primary speed-reducing mechanism. This leads to a problem in that the size of the primary speed-reducing mechanism in the vehicle width direction becomes large, such that the primary speed-reducing mechanism may protrude in the vehicle width direction.
In order to adopt a secondary speed-reducing mechanism of a shaft-drive type, a set consisting of a primary speed-reducing mechanism and a secondary speed-reducing mechanism needs to be developed for each individual model of a two-wheeled electric vehicle. This results in the problem of an increased development cost for the two-wheeled electric vehicle. In a shaft-drive type, the propeller shaft linearly extends in parallel to the rotation axis direction of the gear in the primary speed-reducing mechanism. Moreover, the drive bevel gear at the rear end of the propeller shaft, which extends linearly in parallel to the rotation axis direction of the gear, needs to be aligned with the driven bevel gear on a side of the rear wheel. Therefore, in the case in which a secondary speed-reducing mechanism of a shaft-drive type is adopted, substantial constraints are imposed on the layout of the electric motor, the primary speed-reducing mechanism, and the secondary speed-reducing mechanism.